If there is a difference I have never heard what it was. Some of the earliest production ZL1 Camaros have the '053' casting.
Chevrolet produced a few varieties of aluminum big-block castings back in the day. The earliest [1967] were race parts only, provided to Jim Hall/Chaparral Cars for Can-Am racing. For 1968 McLaren was included. These blocks were significantly different from later production blocks, probably had 0- casting numbers. At this time the term 'ZL1' did not exist. When Chevy decided to offer an aluminum block option for the L-88 Corvette it was assigned option code ZL1. It only applies to castings 3946052 & 053. The latest true ZL1 block castings were produced July-August 1969. Later [1971-1972] Chevy did another aluminum bb casting exclusively for racing.
Be aware that in the late '80s Chevy ran off a number of 3946052/3 castings on the original tooling. They used a different, much darker alloy that does not match the older heads. Some of these blocks are around, machined and raw castings.
http://www.camaros.org/copo.shtml